The present invention relates to optical sonsors used, for example, in an automatic cash receiving and dispensing machine and more particularly to a level adjusting device for the optical sensors used in discriminating the bills.
Usually an automatic cash receiving and dispensing machine used in a bank is provided with optical sensors for discriminating whether a bill is genuine by detecting the presence or absence of a bill in a particular section in the machine, the length of the bill, and the optical transmission pattern or optical reflecting pattern of the bill.
A prior art of such a discriminating device is disclosed in Japanese Laid-Open Patent Publication No. 6597/1976 (hereinafter referred to as "prior document"). In this discrimination device, an analogue detecting signal (for example, the wave-shaped signal shown in FIG. 2 of the prior document) from a photodetecting element such as a phototransistor is compared with a predetemined threshold, and when the analogue detecting signal exceeds the threshold an "H (high)" level signal is obtained and when it is less than the threshold an "L (low)" level signal is obtained and the resultant digital signals are utilized for a subsequent process.
However, in optical sensors which utilize phototransistors and the like, the detected level sometimes varies because of a change with time in the characteristics of the light emitting and receiving elements, deviation in voltage of the power source, and deviation in the detecting level due to a deposit of dust on the sensors. Accordingly, to prevent malfunction of the device, it is required to adjust the amount of light emmission of the light emitting element, or sensitivity of the light receiving element or the threshold, or to remove the dust from the sensors.
The automatic cash receiving and dispensing machine for use in a bank is also provided with an optical sensor intended to discriminate between the presence or absence of a bill and passage of a bill in a particular section in the machine. In general, the presence or absence of a bill is determined by detecting the output level of a light receiving element (e.g. a phototransistor) receiving the light emitted from a light emitting element (light source) and by confirming whether the output level exceeds a reference value (hereinafter referred to as the "threshold").
However, in an optical sensor utilizing the phototransistor etc., the detected level sometimes varies because of a change with time in the characteristics of the light emitting and receiving elements, deviation in voltage of the power source, and deviation in the detecting level due to the deposit of dust on the sensors. Accordingly, it is often required to adjust the sensors or to remove the dust therefrom in response to the level deviations.
A prior art means for compensating for the level deviations of the optical sensors is disclosed in Japanese Utility Model Application No. 157698/1984 (Japaneses Laid-Open Utility Model Publication No. 72689/1986) which was filed by the present applicant under the title "Device for controlling optical sensors". In this device, a dummy set of optical sensors is arranged at a position which is not a part of the transport path of the object (e.g. a bill) to be detected and the change with time in the light receiving characteristics is compensated for by adjusting the threshold in response to the variation of the light receiving characteristics of the dummy set of optical sensors.
However, since the device of the prior art is gaged on the premise that the dummy optical sensors change with time under the same conditions as those to be compensated, it is occasionally impossible to surely carry out the compensational operation owing to differences in the characteristics of the optical sensors or differences in the amount of dust.